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epoxy composites
Materials and Design 45 (2013) 510–517
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Materials and Design journal homepage: www.elsevier.com/locate/matdes
The effect of functionalization on the viscoelastic behavior of multi-wall carbon nanotube/epoxy composites Arash Montazeri ⇑ Department of Chemistry, Islamic Azad University, Tonekabon Branch, Tonekabon, Iran
a r t i c l e
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Article history: Received 13 July 2012 Accepted 6 September 2012 Available online 18 September 2012 Keywords: A. Carbon nanotube B. Viscoelastic properties E. Functionalization
a b s t r a c t Functionalized multi-walled carbon nanotubes with hydroxyl groups (MWNT-OH) and non-functionalized MWNT were used to fabricate MWNT/epoxy composite samples by sonication technique. The viscoelastic properties of the composite samples were evaluated by performing dynamic mechanical thermal analysis (DMTA) test. The results showed that addition of nanotubes to epoxy had significant effect on the viscoelastic properties. Samples containing functionalized nanotubes showed a stronger influence on Tg in comparison to composite samples containing similar amount of non-functionalized nanotubes. The viscoelastic behavior was modeled by plotting the COLE–COLE diagram using the results of DMTA test. There was a good agreement between the Perez model and the viscoelastic behavior of the composite specimen. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction A new allotropy of carbon materials, carbon nanotubes (CNTs) have attracted much interest in the field of CNT/polymer composites, and these materials hold the promise of delivering excellent mechanical properties and multi-functional characteristics [1,2]. Due to their fibrous shape, outstanding mechanical properties and a large specific surface area, CNT-based nanocomposites can be expected to show significantly improved mechanical characteristics as compared to the pure matrix material. This requires successfully performing the critical issues of dispersing as well as surface functionalization. The complete break-up of the agglomerates, the homogenous distribution of the exfoliated nanotubes and their integration into the molecular structure of the matrix are requirements for manufacturing of high quality nanocomposites and the development of their potential [3–5]. Significant research efforts have been devoted to improving the dispersion of CNTs in a polymer matrix. There are two approaches: the mechanical dispersion methods and the surface modification of CNTs based on chemical and physical methods. The mechanical methods include typically ultrasonication in a bath or using a probe sonicator [6,7], high shear mixing in a solvent [8], calendaring and ball milling [9], as well as combined methods in series or parallel. The high energy of ultra sonication often results in damage and breakage of CNTs into smaller lengths [10], which is considered to be a major disadvantage. The chemical methods are aimed at creating surface functionalities on CNTs, thereby improv⇑ Tel.: +98 9111933562; fax: +98 1924274415. E-mail address: [email protected] 0261-3069/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2012.09.013
ing their chemical compatibility/interactions with a polymer or solvent, leading to enhanced dispersion. In general, the viscoelastic properties were found to depend strongly on the degree of CNT dispersion in the polymer matrix. However, quantitative characterization of CNT dispersion is a difficult task [11]. The best indirect method is to perform the dynamic mechanical thermal analysis test. In the field of polymers the study of viscoelastic behavior is very important because other properties are related to this behavior [12]. In this study, we are interested particularly in the viscoelastic behavior of nanocomposite samples. One way to study the viscoelastic behavior of polymer is modeling. Modeling based on COLE–COLE diagram is one of the methods to study the viscoelastic behavior of polymers. This modeling method can give us the predictive information about elastic and viscous properties independently [13]. Nanotubes have already been utilized to improve the viscoelastic and/or thermo-mechanical properties of epoxy matrix. Gong et al. [14] produced the epoxy based nanocomposites by using surfactant modified MWNT and investigated thermo-mechanical properties. It was shown that the incorporation of nanotubes led to an improvement in storage modulus with the use of only 1 wt.% MWNT in the composite sample. Gojny et al. [15] reported the effect of MWNT-NH2 on the thermo-mechanical properties of the MWNT/epoxy composites. The results indicated an improvement in the storage modulus in rubbery state as well as loss modulus values. Miyagawa and Drzal [16] reported that the storage modulus of the epoxy-based nanocomposites containing fluorinated SWNT increased, but the Tg decreased linearly with an increase in the amounts of CNTs. Abdalla et al. [17] showed that
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using carboxylic and fluorinated nanotubes the storage modulus in the glassy state and the rubbery plateau modulus were higher compared to the neat epoxy. Lee et al. [18] modified the MWNT with fluorination treatment to improve dispersion in the epoxy resin. The results showed an increase in the storage and loss modulus values with the increase in the percentage of MWNT. Fidelus [19] studied the effect of MWNT and single walled-carbon nanotube (SWNT) in LY 564 epoxy system. A significant reduction in glass transition temperature was observed. In addition, an increase in the tensile impact strength was obtained in the 0.5 wt.% MWNT/ epoxy composite sample. The realization of nanotube-reinforced epoxy resin can only be achieved by solving following two main problems: one is the lack of interfacial adhesion, which is critical for load transfer in the composite samples and the other is the poor dispersion of nanotubes in the epoxy matrix. These problems can be overcome to a large extend by functionalization treatment of the nanotubes. In order to have a strong interfacial bonding between the nanotubes and the polymer matrix, it is necessary to perform the surface treatment of nano-particles before mixing them with polymer matrix. For example, in an earlier study improved viscoelastic properties has been achieved by carboxylic and amino-functionalization of the MWNT. However, there has been no systematic investigation on the effect of MWNT-bound hydroxylic groups on the viscoelastic properties and also the modeling of the behavior in MWNT/epoxy composite samples. The major objective in this work was to study the viscoelastic properties of epoxy composite samples containing various amounts of multi walled carbon nanotubes. The Perez model was used to study the viscoelastic behavior. 2. Materials and methods 2.1. Nanotubes and polymer material The multi walled carbon nanotubes used in this study were supplied by Hurricane Co. The average length and the diameter of untreated MWNT were 8.5 lm and 20–50 nm respectively. The Hydroxyl treated MWNT had similar diameter but an average length of 2 lm. Low viscosity epoxy resin Ly564 (Araldite) and Hy560 hardener were supplied by Huntsman Co. The resin and the hardener were based-on diglycidyl ether bisphenol-A and polyamine, respectively. 2.2. Preparation of nanocomposites MWNT (0.1, 0.5, 1, 1.5 and 2 wt.%) were mixed with epoxy. The mixture was then sonicated (Bandelin HD3200, 20 kHz) for 2.5 h at 60% amplitude. After sonication, the hardener was added to the mixture and stirred for 15 min at 150 rpm. Air bubbles were removed by placing the mixture under vacuum. The bubble free mixture was then cast on a mold and cured at 60 °C for 1 h followed by 110 °C for 2 h.
LX 30 at 20 kV and S4700 at 0.6 kV). The surface chemical reaction on the MWNT was investigated by Fourier transform infrared spectroscopy (FT-IR, Shimadzu IR solution-8400) in the range of 4000– 500 cm1. 3. Results and discussion 3.1. Nanotube characterization The chemical and structural nature of the nanotubes after modification was characterized using infrared spectroscopy. Fig. 1 shows a typical FT-IR spectrum of the pristine MWNT and Hydroxyl-MWNT. The presence of the characteristic band (the –OH stretching) at 3200–3600 cm1 indicates the generation of groups such as (–OH) on the surface of MWNT. The presence of such functional groups has proven to be beneficial for MWNT/epoxy resin interfacial interaction. In order to study the possible chemical interaction between the nanotube and epoxy, the FT-IR spectra of neat epoxy and the csample containing 2 wt.% MWNT-OH after 2.5 h sonication in epoxy were analysed. Fig. 2 shows the FT-IR spectra for DGEBA epoxy along with 2 wt.% MWNT-OH. As shown in this figure, no extra band was observed in the nanotube containing sample, which indicates the presence of hydrogen bond at the interface between hydroxyl groups of nanotubes and epoxy. 3.2. Morphology characterization The state of nanotube dispersion in epoxy matrix has been characterized by SEM. Figs. 3 and 4 indicate the SEM images obtained from the specimens broken by tensile test. Fig. 3 shows the epoxy and composites fabricated with different non-functionalised nanotube contents, while Fig. 4 shows the epoxy composites reinforced with hydroxyl-functionalised nanotubes. As indicated in these figures, the fracture surface of the neat epoxy is flat and smoothes (Fig. 3a). On the other hand, the fracture surface of nanocomposite samples was significantly rougher than the neat epoxy, evidencing typical brittle fracture behavior (Fig. 3b). Micrometric CNTs agglomerations were also observed on the fracture surface of nanocomposite samples. With the increase in non-functionalized MWNT content, the number and the size of these aggregates grew, indicating a poorer dispersion state of MWNT in the epoxy matrix (Fig. 3c and d). Agglomerates act as stress concentrators, distort crack propagation and therefore increase surface roughness. The fracture surface of the nanocomposites reinforced with 0.5 wt.% MWNT-OH (Fig. 4a) seems totally homogeneous. Comparing
2.3. Analytical methods All samples were mechanically polished to minimize the influence of surface flaw, mainly the porosity. The viscoelastic behavior was investigated by performing dynamic-mechanical thermal analysis using a DMA Netzsch 242 machine. For these measurements, rectangular specimens of 60 mm length, 9 mm width, and 2.5 mm thickness were prepared. The tests were performed in three point bending mode at a frequency of 1 Hz in the temperature range of 90 to 140 °C temperature range with a heating rate of 3 °C/min (ASTM: D5023-01). The cryogenic fracture surface analysis was performed by scanning electron microscope (model
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Fig. 1. FT-IR spectrum of the hydroxyl-functionalized MWNT.
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Fig. 2. FT-IR spectrum of mixture of DGEBA epoxy with 2 wt.% MWNT-OH.
Fig. 3c with Fig. 4a–c indicates that functionalized nanotubes exhibit relatively good dispersion in the matrix resin blend. Good dispersion of nanotubes in the polymer matrix reduces the stress concentration and enhances mechanical properties. The micrographs in high magnification give information about the adhesion and the distribution of nanotubes in the resin matrix. As indicated in Fig. 5, the flat fracture surfaces are characterized by the presence of voids. The formation of these voids may be associated with pull-out of nanotubes from the epoxy matrix. Pull-out of non-functionalized MWNT and a cavity of around 50–100 nm are observed between the nanotubes (Fig. 5a and b). Therefore nanotubes, with weak interface were pulled-out and the mechanical properties under tensile loading showed little change. 3.3. Viscoelastic properties of the resulting nanocomposites DMTA tests are powerful tools for investigation of viscoelastic properties. The main results of DMTA properties of neat epoxy resin and reinforced with different contents of non-functionalized
and hydroxyl-functionalized MWNT are summarized in Table 1.The storage modulus (E0 ) determined at 1 Hz frequency, was plotted as a function of temperature for epoxy and nanocomposite samples (Fig. 6). Two relaxations are observed. A main transition, a, in the high temperature region, is associated with the glass transition. Another relaxation, b, is below 0 °C. The addition of non-functionalized and hydroxyl-functionalized nanotubes into the polymer system has considerable effect on the glassy state. This is due to the stiffening effect of MWNT and interfacial interactions along a huge interfacial area between the CNTs and the polymer matrix [19]. The glassy storage modulus at room temperature increases with the CNTs up to some maximum value. Higher contents cause a decrease in modulus. It seems that with the increase in the agglomerate size at high nanotube content, the molecular motion and the movement of chains become easier. The optimum percentage, at which modulus is highest, is 0.5 wt.% for both types of nanotube. The glassy storage modulus at room temperature is directly related to Young’s modulus and increases with the introduction of MWNT. The maximum modulus values for the composite sample containing 0.5 wt.% MWNT and MWNT-OH were 4296 and 3880 MPa respectively, which is a 46% and 31.5% increase with respect to the neat epoxy sample at 25 °C. Chen et al. [20] reported a 10% increase in storage modulus at 25 °C with 0.57 wt.% oxidized MWNT in epoxy composite. Prolongo et al. [21] investigated the effect of various percentages of non-functionalized MWNT and MWNT-NH2 on the thermo-mechanical properties of the MWNT/ epoxy composites. The results indicated that the storage modulus in glassy state of 0.5 wt.% MWNT and MWNT-NH2 improved by 4% and 15.2% at 30 °C respectively. In the present work, the storage modulus values in the glassy state for MWNT-OH at 0.5 wt.% and 1 wt.% are lower than that of MWNT. According to information provided by the manufacturer of CNTs (Hurricane Co.), the aspect ratio of MWNT with hydroxyl-functional groups is lower than those without any surface treatment. Therefore, the addition of nanotube into the epoxy resin is expected to result in relatively lower storage modulus in glassy states. Avilés et al. [22] reported the effect of two different nanotubes on the thermo-mechanical properties. The results showed that improvement in the storage modulus were greater for composite samples containing nanotubes with larger
Fig. 3. SEM images of the fracture surface epoxy and MWNT/epoxy nanocomposites: (a) epoxy, (b) 0.1 wt.% MWNT, (c) 0.5 wt.% MWNT and (d) 2 wt.% MWNT.
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Fig. 4. SEM images of the fracture surface MWNT-OH/epoxy nanocomposites: (a) 0.5 wt.% MWNT-OH, (b) 1 wt.% MWNT-OH, and (c) 2 wt.% MWNT-OH.
Fig. 5. SEM images at high magnification: (a) 1 wt.% MWNT, 60,000 and (b) 0.5 wt.% MWNT, 110,000.
aspect ratios. However, Schulte [19] and co-worker obtained opposite results. They indicated that the incorporation of amino-functionalized MWNT with low aspect ratio into the vinyl esterpolyester provided higher storage modulus in glassy state. It was noted that the storage modulus of composites increased at higher temperatures; in the rubbery state between 105 °C and 120 °C (Fig. 7). This behavior can be explained in terms of an inter-
Table 1 Summary of key DMTA properties. Material
Tb (°C)
Tg (°C)
E0 at 25 °C (MPa)
E0R at 120 °C (MPa)
Epoxy 0.1 wt.% MWNT 0.5 wt.% MWNT 1 wt.% MWNT 1.5 wt.% MWNT 2 wt.% MWNT 0.5 wt.% MWNTOH 1 wt.% MWNT-OH 2 wt.% MWNT-OH
42.2 42.2 42.2 42.2 42.2 42.2 42.2
89 91 88 88 88.5 91 92
2945 3650 4315 3780 3253 3253 3859
206 179 227 227 179 175 352
42.2 42.2
93.5 95.5
3455 3624
325 322
Fig. 6. Storage modulus for epoxy and nanocomposites with different weight percents at 1 Hz frequency.
action between the MWNT and the epoxy due to high surface area. The rubbery modulus was the highest with 0.5 wt.% MWNT for
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Fig. 7. Rubbery modulus for epoxy and nanocomposite samples with different weight percents of MWNT.
both types of nanotubes. The rubbery modulus for MWNT and MWNT-OH composites was 9.1% and 71.8% higher than that of neat epoxy at 120 °C respectively. A strong increase of storage modulus, especially above Tg was observed for functionalized nanotube used in this study. In fact in this state, the molecular motion and the amplitude of this motion are very high and the macromolecule is not practically in contact with particles. So, the interaction between hydroxyl groups and epoxy prevents the reduction of rubbery modulus at high temperatures. On the other hand, the hydroxyl groups on the surface of nanotubes cause a topological hindrance between MWNT, leading to a better dispersion in epoxy matrix (Fig. 8a and b). In contrast to the enormous research on the elastic properties of CNT/polymer nanocomposites, relatively less attention has been paid to their damping mechanisms and ability. Damping indicates the energy converted into heat and can thus be used as a measurement of viscous component or unrecoverable oscillation energy dissipated per cycle. As such, the damping properties of nanocomposites, such as loss factor and damping ratio, are essential design parameters for many engineering applications. High damping properties can be achieved in nanocomposites by taking advantage of the interfacial friction between the nano-fillers and the polymer matrix [23–25]. The damping capacity of the nanocomposites is usually measured by increases in loss modulus [26,27]. Fig. 9 shows the loss modulus (E00 ) for neat epoxy and nanocomposites. As indicated in this figure, the addition of MWNT and MWNT-OH
increase the damping capacity. However, composites containing hydroxyl functionalized nanotubes show lower peak values compared to those with non-functionalized nanotubes. A weak interfacial adhesion plays an important role in enhancing the damping properties of CNT nanocomposites, although it is detrimental to the mechanical properties of CNT/polymer nanocomposites. In contrast, a stronger interfacial bond arising from functionalization decreases the damping response [28]. The loss modulus initially increased at 1 wt.% MWNT, which was followed by a continuous decrease in the peak height at higher MWNT values. The dispersed nanotubes dissipate energy due to resistance against viscoelastic deformation of the surrounding epoxy matrix. The decrease of E00 at higher nanotube contents can be interpreted by an increasing susceptibility of agglomeration, leading to less energy dissipation in the system under viscoelastic deformation [29]. Fig. 10 shows the tangent delta (tan d) vs. temperature curves. The glass transition temperature values of the composite with MWNT and MWNT-OH obtained from the maximum peak are collected in Table 1. At 2 wt.% MWNT the Tg increases only 1 °C, but the addition of 1 wt.% and 2 wt.% MWNT-OH to the resin blend increases Tg by 4 and 6 °C respectively. The mobility of the polymer matrix around the nanotubes is reduced due to the presence of nanotubes. Hydrogen atoms at the –OH groups of functionalized MWNT may form hydrogen bonds with epoxy groups. Hence, the affinity of MWNT-OH to epoxy matrix is expected to exceed that of the MWNT. This affinity reduces the mobility of the epoxy matrix around the MWNT-OH and leads to the observed increase in thermal stability. This effect will basically appear around and above Tg, due to the limited potential movement of the polymeric matrix. It is shown that incorporation of 1 wt.% oxidized MWNT and amino-functionalized MWNT into the epoxy increased Tg by about 6 and 11 °C respectively [30], While Prolongo et al. [21] indicated that the addition of 1 wt.% amino-functionalized MWNT into epoxy matrix did not have any effect on the magnitude of Tg values. The results obtained in this study shows that the Functionalization of MWNT with hydroxyl groups and hydrogen bonding can improve the Tg similar to other groups with covalent bonds. It is interesting to note that the acid-modified and amino-functionalized MWNT contain covalent bond which is stronger than the hydrogen bond. It is well known that the a relaxation is associated with conformational crank shafting movements of the main chain, whereas the b transitions are associated with movements of the side groups. The assignment of the b relaxation to molecular segment motion depends on the chemical structure of the compounds. There were no significant differences between the temperatures at maximum tan delta peak (Tb) (Fig. 11). Therefore, it can be concluded that
Fig. 8. Schematic of dispersion of (a) MWNT and (b) MWNT-OH in the epoxy matrix.
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Fig. 9. Loss modulus for epoxy and nanocomposite samples with different weight percents.
Fig. 10. Tan d curves for epoxy and nanocomposite samples with different weight percents.
the nanotubes did not have any effect on the movement of the side groups in the composite specimen. Finally, DMTA results can also provide information about the homogeneity of nanocomposites. The peak factor, U, is defined as the full width at half maximum of the tan d peak divided by its height, and it can be used to assess the homogeneity of the epoxy network. Broadening of tan d peak at high temperature could be the result of interaction of epoxy network with the nano-reinforcements [21]. However, this value remains approximately constant for non-functionalized MWNT (Fig. 12). For MWNT-OH/epoxy composite, the peak factor increases with increasing MWNT weight percent, and it exhibits a broadened tan d peak on the high temperature side of the DMTA profile. The higher peak factor for the functionalized MWNT composites is indicative of lower crosslink density and greater heterogeneity, which suggests interaction of nanotubes with epoxy matrix [31]. 3.3.1. Modeling of the viscoelastic behavior The Perez model was applied to investigate the viscoelastic behavior. This model usually gives a good fit of COLE–COLE plots. The model leads to the following equation for the complex modulus [32]:
E ¼ E1
E1 E0 1 þ ðixsÞv þ Q ðixsÞv
0
ð1Þ
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Fig. 11. The b relaxation of the tan delta curves for epoxy and nanocomposites with different weight percent sin the 70 to 10 °C temperature range.
Fig. 12. Effect of nanotubes content on peak factor of epoxy.
where E0 and E1 are the relaxed and unrelaxed modulus respectively. Each transition is characterized by a pair of (E0, E1). In the case of glass transition, E0 is the equilibrium rubbery modulus, and E1 is the modulus at the glassy regions just above a transition. v and v0 are related to the slopes dE00 /dE’ as well as a relaxation. Finally, Q, the function of concentration of quasi-point defects, is related to maximum value of E00 and increases with decreasing Q. The program for this model has been coded in MATLAB environment. Fig. 13a–e shows the COLE–COLE plots for epoxy and nanocomposite samples. Their COLE–COLE plots are generally nonsymmetrical. The values of parameters of the model for epoxy and nanocomposites are shown in Table 2. From the results presented in this table, we can draw the following conclusions. The parameter v shows molecular motion and the intensity of the effects of correlation involved during expansion of the Somigliana dislocation (smd) [32]. The values of v for MWNT/epoxy composites seem to show very little variation. While, the addition of MWNT-OH into the epoxy matrix decreases the values of v. An increase in the value of v indicates weaker molecular motion. The parameter Q is related to the maximum value of E00 which increases with decreasing Q. A decrease in the value of Q is shown with addition of non-functionalized MWNT to the epoxy. With the decrease in the value of Q, the loss modulus increases and an improvement is observed in the damping properties (Fig. 7).
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Fig. 13. COLE–COLE plots for (a) epoxy, (b) 0.5 wt.% MWNT, (c) 0.5 wt.% MWNT-OH, (d) 2 wt.% MWNT and (e) 2 wt.% MWNT-OH.
Table 2 The parameters used for the calculations of plots shown in Fig. 13a–e. Material
E0 (MPa)
E1 (MPa)
v0
X
Q
sa
Epoxy 0.1 wt.% MWNT 0.5 wt.% MWNT 1 wt.% MWNT 1.5 wt.% MWNT 2 wt.% MWNT 0.5 wt.% MWNT-OH 1 wt.% MWNT-OH 2 wt.% MWNT-OH
2600 2950 3270 3160 2800 2900 3400 3250 3400
210 200 220 230 175 210 230 270 280
0.60 0.61 0.60 0.61 0.60 0.59 0.57 0.53 0.51
0.20 0.20 0.20 0.19 0.20 0.19 0.17 0.16 0.14
0.50 0.45 0.40 0.40 0.46 0.47 0.42 0.45 0.40
0.16 0.16 0.16 0.16 0.16 0.16 0.16 0.16 0.16
The parameter v0 accounts for the difficulty with which local shearing occurs and it is the most important parameter in Perez model. When v0 decreases, the movement of polymer chains become slower. As soon as a local shearing takes place, the molecular orientation resulting from it makes molecular mobility more difficult. In other words, v decreases rapidly when v0 is low. In a cross linked system the value of v0 is still low. For example the values of v0 for the neat epoxy is 0.6. These values are in good agreement with the values reported for DGEBA–DDM cross linked system [32]. The addition of non-functionalized MWNT to epoxy had no effect on the values of v0 . In other words, in a region, local shearing occurs easily and Tg decreases. But, the addition of MWNT-OH to epoxy decreases the values of v0 . In fact in a region that the molec-
A. Montazeri / Materials and Design 45 (2013) 510–517
ular motion and local shearing are very high and so the macromolecule is not practically in contact with nanotube, interaction between hydroxyl groups and epoxy prevents the movement of polymer chains and the reduction of glass transition temperature. The parameters of Perez model indicates a good correlation between viscoelastic experimental results and the model used. 4. Conclusions Epoxy nanocomposite samples containing different weight percentages of non-functionalized and functionalized MWNT were fabricated in this study. The viscoelastic properties of the samples were evaluated. The hydroxyl groups on the MWNT surface were successfully increased the dispersion in the epoxy matrix. The dispersion degree of MWNT-OH significantly affects the improvement of viscoelastic properties. The addition of 0.5 wt.% MWNT and MWNT-OH to the epoxy matrix increased the storage modulus by 46% and 31.5% at room temperature. The rubbery modulus for 0.5 wt.% MWNT and MWNT-OH/epoxy composite was 9.1% and 71.8% higher than that of neat epoxy at 120 °C respectively. The weak interface between epoxy matrix and MWNT increased the loss modulus and damping of MWNT/epoxy composite specimen. It was demonstrated that the a-relaxation temperature increased with the addition of MWNT-OH. It seems that the hydrogen bonding between –OH groups with epoxy to be effective in the increase of viscoelastic properties. The addition of nanotubes did not have any effect on the b relaxation temperature. The parameters of Perez model showed a good correlation between viscoelastic experimental results and the model used. Acknowledgements The authors would like to express their gratitude to the personnel at Laboratoired’ingenierie des materiaux, ENSAM-Paris for conducting DMTA tests and SEM images. References [1] IiJima S. Hellical microtubules. Nature 1991;354:56–8. [2] IiJima S, Icihashi T. Single-shell carbon nanotubes of 1-nm diameter. Nature 1993;363:603–5. [3] Bethune DC, Kiang CH, Devries MS. Cobalt-catalyzed growth of carbon nanotubes with single-atomic-layer walls. Nature 1993;363:605–7. [4] Thostenson ET, Ren Z, Chow TW. Advances in the science and technology of carbon nanotubes and their composites: a review. Compos Sci Technol 2001;61:1899–912. [5] Jin Z, Promoda KPG. Dynamic mechanical behavior of melt-processed multiwalled carbon nanotube/poly (methacrylate) composite. Chem Phys Lett 2001;337:43–7. [6] Liao YH, Marietta-Tondin O, Liang Z, Zhang C, Wang B. Investigation of the dispersion process of SWNT/SC-15 epoxy resin nanocomposite. Mater Sci Eng 2004;A385:175–81. [7] Spitalsky´ Z, Krontiras CA, Georga SN, Galiotis C. Effect of oxidation treatment of multiwalled carbon nanotubes on the mechanical and electrical properties of their epoxy composites. Composites Part A 2009;40:778–83.
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Contents lists available at SciVerse ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
The effect of functionalization on the viscoelastic behavior of multi-wall carbon nanotube/epoxy composites Arash Montazeri ⇑ Department of Chemistry, Islamic Azad University, Tonekabon Branch, Tonekabon, Iran
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Article history: Received 13 July 2012 Accepted 6 September 2012 Available online 18 September 2012 Keywords: A. Carbon nanotube B. Viscoelastic properties E. Functionalization
a b s t r a c t Functionalized multi-walled carbon nanotubes with hydroxyl groups (MWNT-OH) and non-functionalized MWNT were used to fabricate MWNT/epoxy composite samples by sonication technique. The viscoelastic properties of the composite samples were evaluated by performing dynamic mechanical thermal analysis (DMTA) test. The results showed that addition of nanotubes to epoxy had significant effect on the viscoelastic properties. Samples containing functionalized nanotubes showed a stronger influence on Tg in comparison to composite samples containing similar amount of non-functionalized nanotubes. The viscoelastic behavior was modeled by plotting the COLE–COLE diagram using the results of DMTA test. There was a good agreement between the Perez model and the viscoelastic behavior of the composite specimen. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction A new allotropy of carbon materials, carbon nanotubes (CNTs) have attracted much interest in the field of CNT/polymer composites, and these materials hold the promise of delivering excellent mechanical properties and multi-functional characteristics [1,2]. Due to their fibrous shape, outstanding mechanical properties and a large specific surface area, CNT-based nanocomposites can be expected to show significantly improved mechanical characteristics as compared to the pure matrix material. This requires successfully performing the critical issues of dispersing as well as surface functionalization. The complete break-up of the agglomerates, the homogenous distribution of the exfoliated nanotubes and their integration into the molecular structure of the matrix are requirements for manufacturing of high quality nanocomposites and the development of their potential [3–5]. Significant research efforts have been devoted to improving the dispersion of CNTs in a polymer matrix. There are two approaches: the mechanical dispersion methods and the surface modification of CNTs based on chemical and physical methods. The mechanical methods include typically ultrasonication in a bath or using a probe sonicator [6,7], high shear mixing in a solvent [8], calendaring and ball milling [9], as well as combined methods in series or parallel. The high energy of ultra sonication often results in damage and breakage of CNTs into smaller lengths [10], which is considered to be a major disadvantage. The chemical methods are aimed at creating surface functionalities on CNTs, thereby improv⇑ Tel.: +98 9111933562; fax: +98 1924274415. E-mail address: [email protected] 0261-3069/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2012.09.013
ing their chemical compatibility/interactions with a polymer or solvent, leading to enhanced dispersion. In general, the viscoelastic properties were found to depend strongly on the degree of CNT dispersion in the polymer matrix. However, quantitative characterization of CNT dispersion is a difficult task [11]. The best indirect method is to perform the dynamic mechanical thermal analysis test. In the field of polymers the study of viscoelastic behavior is very important because other properties are related to this behavior [12]. In this study, we are interested particularly in the viscoelastic behavior of nanocomposite samples. One way to study the viscoelastic behavior of polymer is modeling. Modeling based on COLE–COLE diagram is one of the methods to study the viscoelastic behavior of polymers. This modeling method can give us the predictive information about elastic and viscous properties independently [13]. Nanotubes have already been utilized to improve the viscoelastic and/or thermo-mechanical properties of epoxy matrix. Gong et al. [14] produced the epoxy based nanocomposites by using surfactant modified MWNT and investigated thermo-mechanical properties. It was shown that the incorporation of nanotubes led to an improvement in storage modulus with the use of only 1 wt.% MWNT in the composite sample. Gojny et al. [15] reported the effect of MWNT-NH2 on the thermo-mechanical properties of the MWNT/epoxy composites. The results indicated an improvement in the storage modulus in rubbery state as well as loss modulus values. Miyagawa and Drzal [16] reported that the storage modulus of the epoxy-based nanocomposites containing fluorinated SWNT increased, but the Tg decreased linearly with an increase in the amounts of CNTs. Abdalla et al. [17] showed that
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using carboxylic and fluorinated nanotubes the storage modulus in the glassy state and the rubbery plateau modulus were higher compared to the neat epoxy. Lee et al. [18] modified the MWNT with fluorination treatment to improve dispersion in the epoxy resin. The results showed an increase in the storage and loss modulus values with the increase in the percentage of MWNT. Fidelus [19] studied the effect of MWNT and single walled-carbon nanotube (SWNT) in LY 564 epoxy system. A significant reduction in glass transition temperature was observed. In addition, an increase in the tensile impact strength was obtained in the 0.5 wt.% MWNT/ epoxy composite sample. The realization of nanotube-reinforced epoxy resin can only be achieved by solving following two main problems: one is the lack of interfacial adhesion, which is critical for load transfer in the composite samples and the other is the poor dispersion of nanotubes in the epoxy matrix. These problems can be overcome to a large extend by functionalization treatment of the nanotubes. In order to have a strong interfacial bonding between the nanotubes and the polymer matrix, it is necessary to perform the surface treatment of nano-particles before mixing them with polymer matrix. For example, in an earlier study improved viscoelastic properties has been achieved by carboxylic and amino-functionalization of the MWNT. However, there has been no systematic investigation on the effect of MWNT-bound hydroxylic groups on the viscoelastic properties and also the modeling of the behavior in MWNT/epoxy composite samples. The major objective in this work was to study the viscoelastic properties of epoxy composite samples containing various amounts of multi walled carbon nanotubes. The Perez model was used to study the viscoelastic behavior. 2. Materials and methods 2.1. Nanotubes and polymer material The multi walled carbon nanotubes used in this study were supplied by Hurricane Co. The average length and the diameter of untreated MWNT were 8.5 lm and 20–50 nm respectively. The Hydroxyl treated MWNT had similar diameter but an average length of 2 lm. Low viscosity epoxy resin Ly564 (Araldite) and Hy560 hardener were supplied by Huntsman Co. The resin and the hardener were based-on diglycidyl ether bisphenol-A and polyamine, respectively. 2.2. Preparation of nanocomposites MWNT (0.1, 0.5, 1, 1.5 and 2 wt.%) were mixed with epoxy. The mixture was then sonicated (Bandelin HD3200, 20 kHz) for 2.5 h at 60% amplitude. After sonication, the hardener was added to the mixture and stirred for 15 min at 150 rpm. Air bubbles were removed by placing the mixture under vacuum. The bubble free mixture was then cast on a mold and cured at 60 °C for 1 h followed by 110 °C for 2 h.
LX 30 at 20 kV and S4700 at 0.6 kV). The surface chemical reaction on the MWNT was investigated by Fourier transform infrared spectroscopy (FT-IR, Shimadzu IR solution-8400) in the range of 4000– 500 cm1. 3. Results and discussion 3.1. Nanotube characterization The chemical and structural nature of the nanotubes after modification was characterized using infrared spectroscopy. Fig. 1 shows a typical FT-IR spectrum of the pristine MWNT and Hydroxyl-MWNT. The presence of the characteristic band (the –OH stretching) at 3200–3600 cm1 indicates the generation of groups such as (–OH) on the surface of MWNT. The presence of such functional groups has proven to be beneficial for MWNT/epoxy resin interfacial interaction. In order to study the possible chemical interaction between the nanotube and epoxy, the FT-IR spectra of neat epoxy and the csample containing 2 wt.% MWNT-OH after 2.5 h sonication in epoxy were analysed. Fig. 2 shows the FT-IR spectra for DGEBA epoxy along with 2 wt.% MWNT-OH. As shown in this figure, no extra band was observed in the nanotube containing sample, which indicates the presence of hydrogen bond at the interface between hydroxyl groups of nanotubes and epoxy. 3.2. Morphology characterization The state of nanotube dispersion in epoxy matrix has been characterized by SEM. Figs. 3 and 4 indicate the SEM images obtained from the specimens broken by tensile test. Fig. 3 shows the epoxy and composites fabricated with different non-functionalised nanotube contents, while Fig. 4 shows the epoxy composites reinforced with hydroxyl-functionalised nanotubes. As indicated in these figures, the fracture surface of the neat epoxy is flat and smoothes (Fig. 3a). On the other hand, the fracture surface of nanocomposite samples was significantly rougher than the neat epoxy, evidencing typical brittle fracture behavior (Fig. 3b). Micrometric CNTs agglomerations were also observed on the fracture surface of nanocomposite samples. With the increase in non-functionalized MWNT content, the number and the size of these aggregates grew, indicating a poorer dispersion state of MWNT in the epoxy matrix (Fig. 3c and d). Agglomerates act as stress concentrators, distort crack propagation and therefore increase surface roughness. The fracture surface of the nanocomposites reinforced with 0.5 wt.% MWNT-OH (Fig. 4a) seems totally homogeneous. Comparing
2.3. Analytical methods All samples were mechanically polished to minimize the influence of surface flaw, mainly the porosity. The viscoelastic behavior was investigated by performing dynamic-mechanical thermal analysis using a DMA Netzsch 242 machine. For these measurements, rectangular specimens of 60 mm length, 9 mm width, and 2.5 mm thickness were prepared. The tests were performed in three point bending mode at a frequency of 1 Hz in the temperature range of 90 to 140 °C temperature range with a heating rate of 3 °C/min (ASTM: D5023-01). The cryogenic fracture surface analysis was performed by scanning electron microscope (model
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Fig. 1. FT-IR spectrum of the hydroxyl-functionalized MWNT.
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Fig. 2. FT-IR spectrum of mixture of DGEBA epoxy with 2 wt.% MWNT-OH.
Fig. 3c with Fig. 4a–c indicates that functionalized nanotubes exhibit relatively good dispersion in the matrix resin blend. Good dispersion of nanotubes in the polymer matrix reduces the stress concentration and enhances mechanical properties. The micrographs in high magnification give information about the adhesion and the distribution of nanotubes in the resin matrix. As indicated in Fig. 5, the flat fracture surfaces are characterized by the presence of voids. The formation of these voids may be associated with pull-out of nanotubes from the epoxy matrix. Pull-out of non-functionalized MWNT and a cavity of around 50–100 nm are observed between the nanotubes (Fig. 5a and b). Therefore nanotubes, with weak interface were pulled-out and the mechanical properties under tensile loading showed little change. 3.3. Viscoelastic properties of the resulting nanocomposites DMTA tests are powerful tools for investigation of viscoelastic properties. The main results of DMTA properties of neat epoxy resin and reinforced with different contents of non-functionalized
and hydroxyl-functionalized MWNT are summarized in Table 1.The storage modulus (E0 ) determined at 1 Hz frequency, was plotted as a function of temperature for epoxy and nanocomposite samples (Fig. 6). Two relaxations are observed. A main transition, a, in the high temperature region, is associated with the glass transition. Another relaxation, b, is below 0 °C. The addition of non-functionalized and hydroxyl-functionalized nanotubes into the polymer system has considerable effect on the glassy state. This is due to the stiffening effect of MWNT and interfacial interactions along a huge interfacial area between the CNTs and the polymer matrix [19]. The glassy storage modulus at room temperature increases with the CNTs up to some maximum value. Higher contents cause a decrease in modulus. It seems that with the increase in the agglomerate size at high nanotube content, the molecular motion and the movement of chains become easier. The optimum percentage, at which modulus is highest, is 0.5 wt.% for both types of nanotube. The glassy storage modulus at room temperature is directly related to Young’s modulus and increases with the introduction of MWNT. The maximum modulus values for the composite sample containing 0.5 wt.% MWNT and MWNT-OH were 4296 and 3880 MPa respectively, which is a 46% and 31.5% increase with respect to the neat epoxy sample at 25 °C. Chen et al. [20] reported a 10% increase in storage modulus at 25 °C with 0.57 wt.% oxidized MWNT in epoxy composite. Prolongo et al. [21] investigated the effect of various percentages of non-functionalized MWNT and MWNT-NH2 on the thermo-mechanical properties of the MWNT/ epoxy composites. The results indicated that the storage modulus in glassy state of 0.5 wt.% MWNT and MWNT-NH2 improved by 4% and 15.2% at 30 °C respectively. In the present work, the storage modulus values in the glassy state for MWNT-OH at 0.5 wt.% and 1 wt.% are lower than that of MWNT. According to information provided by the manufacturer of CNTs (Hurricane Co.), the aspect ratio of MWNT with hydroxyl-functional groups is lower than those without any surface treatment. Therefore, the addition of nanotube into the epoxy resin is expected to result in relatively lower storage modulus in glassy states. Avilés et al. [22] reported the effect of two different nanotubes on the thermo-mechanical properties. The results showed that improvement in the storage modulus were greater for composite samples containing nanotubes with larger
Fig. 3. SEM images of the fracture surface epoxy and MWNT/epoxy nanocomposites: (a) epoxy, (b) 0.1 wt.% MWNT, (c) 0.5 wt.% MWNT and (d) 2 wt.% MWNT.
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Fig. 4. SEM images of the fracture surface MWNT-OH/epoxy nanocomposites: (a) 0.5 wt.% MWNT-OH, (b) 1 wt.% MWNT-OH, and (c) 2 wt.% MWNT-OH.
Fig. 5. SEM images at high magnification: (a) 1 wt.% MWNT, 60,000 and (b) 0.5 wt.% MWNT, 110,000.
aspect ratios. However, Schulte [19] and co-worker obtained opposite results. They indicated that the incorporation of amino-functionalized MWNT with low aspect ratio into the vinyl esterpolyester provided higher storage modulus in glassy state. It was noted that the storage modulus of composites increased at higher temperatures; in the rubbery state between 105 °C and 120 °C (Fig. 7). This behavior can be explained in terms of an inter-
Table 1 Summary of key DMTA properties. Material
Tb (°C)
Tg (°C)
E0 at 25 °C (MPa)
E0R at 120 °C (MPa)
Epoxy 0.1 wt.% MWNT 0.5 wt.% MWNT 1 wt.% MWNT 1.5 wt.% MWNT 2 wt.% MWNT 0.5 wt.% MWNTOH 1 wt.% MWNT-OH 2 wt.% MWNT-OH
42.2 42.2 42.2 42.2 42.2 42.2 42.2
89 91 88 88 88.5 91 92
2945 3650 4315 3780 3253 3253 3859
206 179 227 227 179 175 352
42.2 42.2
93.5 95.5
3455 3624
325 322
Fig. 6. Storage modulus for epoxy and nanocomposites with different weight percents at 1 Hz frequency.
action between the MWNT and the epoxy due to high surface area. The rubbery modulus was the highest with 0.5 wt.% MWNT for
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Fig. 7. Rubbery modulus for epoxy and nanocomposite samples with different weight percents of MWNT.
both types of nanotubes. The rubbery modulus for MWNT and MWNT-OH composites was 9.1% and 71.8% higher than that of neat epoxy at 120 °C respectively. A strong increase of storage modulus, especially above Tg was observed for functionalized nanotube used in this study. In fact in this state, the molecular motion and the amplitude of this motion are very high and the macromolecule is not practically in contact with particles. So, the interaction between hydroxyl groups and epoxy prevents the reduction of rubbery modulus at high temperatures. On the other hand, the hydroxyl groups on the surface of nanotubes cause a topological hindrance between MWNT, leading to a better dispersion in epoxy matrix (Fig. 8a and b). In contrast to the enormous research on the elastic properties of CNT/polymer nanocomposites, relatively less attention has been paid to their damping mechanisms and ability. Damping indicates the energy converted into heat and can thus be used as a measurement of viscous component or unrecoverable oscillation energy dissipated per cycle. As such, the damping properties of nanocomposites, such as loss factor and damping ratio, are essential design parameters for many engineering applications. High damping properties can be achieved in nanocomposites by taking advantage of the interfacial friction between the nano-fillers and the polymer matrix [23–25]. The damping capacity of the nanocomposites is usually measured by increases in loss modulus [26,27]. Fig. 9 shows the loss modulus (E00 ) for neat epoxy and nanocomposites. As indicated in this figure, the addition of MWNT and MWNT-OH
increase the damping capacity. However, composites containing hydroxyl functionalized nanotubes show lower peak values compared to those with non-functionalized nanotubes. A weak interfacial adhesion plays an important role in enhancing the damping properties of CNT nanocomposites, although it is detrimental to the mechanical properties of CNT/polymer nanocomposites. In contrast, a stronger interfacial bond arising from functionalization decreases the damping response [28]. The loss modulus initially increased at 1 wt.% MWNT, which was followed by a continuous decrease in the peak height at higher MWNT values. The dispersed nanotubes dissipate energy due to resistance against viscoelastic deformation of the surrounding epoxy matrix. The decrease of E00 at higher nanotube contents can be interpreted by an increasing susceptibility of agglomeration, leading to less energy dissipation in the system under viscoelastic deformation [29]. Fig. 10 shows the tangent delta (tan d) vs. temperature curves. The glass transition temperature values of the composite with MWNT and MWNT-OH obtained from the maximum peak are collected in Table 1. At 2 wt.% MWNT the Tg increases only 1 °C, but the addition of 1 wt.% and 2 wt.% MWNT-OH to the resin blend increases Tg by 4 and 6 °C respectively. The mobility of the polymer matrix around the nanotubes is reduced due to the presence of nanotubes. Hydrogen atoms at the –OH groups of functionalized MWNT may form hydrogen bonds with epoxy groups. Hence, the affinity of MWNT-OH to epoxy matrix is expected to exceed that of the MWNT. This affinity reduces the mobility of the epoxy matrix around the MWNT-OH and leads to the observed increase in thermal stability. This effect will basically appear around and above Tg, due to the limited potential movement of the polymeric matrix. It is shown that incorporation of 1 wt.% oxidized MWNT and amino-functionalized MWNT into the epoxy increased Tg by about 6 and 11 °C respectively [30], While Prolongo et al. [21] indicated that the addition of 1 wt.% amino-functionalized MWNT into epoxy matrix did not have any effect on the magnitude of Tg values. The results obtained in this study shows that the Functionalization of MWNT with hydroxyl groups and hydrogen bonding can improve the Tg similar to other groups with covalent bonds. It is interesting to note that the acid-modified and amino-functionalized MWNT contain covalent bond which is stronger than the hydrogen bond. It is well known that the a relaxation is associated with conformational crank shafting movements of the main chain, whereas the b transitions are associated with movements of the side groups. The assignment of the b relaxation to molecular segment motion depends on the chemical structure of the compounds. There were no significant differences between the temperatures at maximum tan delta peak (Tb) (Fig. 11). Therefore, it can be concluded that
Fig. 8. Schematic of dispersion of (a) MWNT and (b) MWNT-OH in the epoxy matrix.
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Fig. 9. Loss modulus for epoxy and nanocomposite samples with different weight percents.
Fig. 10. Tan d curves for epoxy and nanocomposite samples with different weight percents.
the nanotubes did not have any effect on the movement of the side groups in the composite specimen. Finally, DMTA results can also provide information about the homogeneity of nanocomposites. The peak factor, U, is defined as the full width at half maximum of the tan d peak divided by its height, and it can be used to assess the homogeneity of the epoxy network. Broadening of tan d peak at high temperature could be the result of interaction of epoxy network with the nano-reinforcements [21]. However, this value remains approximately constant for non-functionalized MWNT (Fig. 12). For MWNT-OH/epoxy composite, the peak factor increases with increasing MWNT weight percent, and it exhibits a broadened tan d peak on the high temperature side of the DMTA profile. The higher peak factor for the functionalized MWNT composites is indicative of lower crosslink density and greater heterogeneity, which suggests interaction of nanotubes with epoxy matrix [31]. 3.3.1. Modeling of the viscoelastic behavior The Perez model was applied to investigate the viscoelastic behavior. This model usually gives a good fit of COLE–COLE plots. The model leads to the following equation for the complex modulus [32]:
E ¼ E1
E1 E0 1 þ ðixsÞv þ Q ðixsÞv
0
ð1Þ
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Fig. 11. The b relaxation of the tan delta curves for epoxy and nanocomposites with different weight percent sin the 70 to 10 °C temperature range.
Fig. 12. Effect of nanotubes content on peak factor of epoxy.
where E0 and E1 are the relaxed and unrelaxed modulus respectively. Each transition is characterized by a pair of (E0, E1). In the case of glass transition, E0 is the equilibrium rubbery modulus, and E1 is the modulus at the glassy regions just above a transition. v and v0 are related to the slopes dE00 /dE’ as well as a relaxation. Finally, Q, the function of concentration of quasi-point defects, is related to maximum value of E00 and increases with decreasing Q. The program for this model has been coded in MATLAB environment. Fig. 13a–e shows the COLE–COLE plots for epoxy and nanocomposite samples. Their COLE–COLE plots are generally nonsymmetrical. The values of parameters of the model for epoxy and nanocomposites are shown in Table 2. From the results presented in this table, we can draw the following conclusions. The parameter v shows molecular motion and the intensity of the effects of correlation involved during expansion of the Somigliana dislocation (smd) [32]. The values of v for MWNT/epoxy composites seem to show very little variation. While, the addition of MWNT-OH into the epoxy matrix decreases the values of v. An increase in the value of v indicates weaker molecular motion. The parameter Q is related to the maximum value of E00 which increases with decreasing Q. A decrease in the value of Q is shown with addition of non-functionalized MWNT to the epoxy. With the decrease in the value of Q, the loss modulus increases and an improvement is observed in the damping properties (Fig. 7).
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Fig. 13. COLE–COLE plots for (a) epoxy, (b) 0.5 wt.% MWNT, (c) 0.5 wt.% MWNT-OH, (d) 2 wt.% MWNT and (e) 2 wt.% MWNT-OH.
Table 2 The parameters used for the calculations of plots shown in Fig. 13a–e. Material
E0 (MPa)
E1 (MPa)
v0
X
Q
sa
Epoxy 0.1 wt.% MWNT 0.5 wt.% MWNT 1 wt.% MWNT 1.5 wt.% MWNT 2 wt.% MWNT 0.5 wt.% MWNT-OH 1 wt.% MWNT-OH 2 wt.% MWNT-OH
2600 2950 3270 3160 2800 2900 3400 3250 3400
210 200 220 230 175 210 230 270 280
0.60 0.61 0.60 0.61 0.60 0.59 0.57 0.53 0.51
0.20 0.20 0.20 0.19 0.20 0.19 0.17 0.16 0.14
0.50 0.45 0.40 0.40 0.46 0.47 0.42 0.45 0.40
0.16 0.16 0.16 0.16 0.16 0.16 0.16 0.16 0.16
The parameter v0 accounts for the difficulty with which local shearing occurs and it is the most important parameter in Perez model. When v0 decreases, the movement of polymer chains become slower. As soon as a local shearing takes place, the molecular orientation resulting from it makes molecular mobility more difficult. In other words, v decreases rapidly when v0 is low. In a cross linked system the value of v0 is still low. For example the values of v0 for the neat epoxy is 0.6. These values are in good agreement with the values reported for DGEBA–DDM cross linked system [32]. The addition of non-functionalized MWNT to epoxy had no effect on the values of v0 . In other words, in a region, local shearing occurs easily and Tg decreases. But, the addition of MWNT-OH to epoxy decreases the values of v0 . In fact in a region that the molec-
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ular motion and local shearing are very high and so the macromolecule is not practically in contact with nanotube, interaction between hydroxyl groups and epoxy prevents the movement of polymer chains and the reduction of glass transition temperature. The parameters of Perez model indicates a good correlation between viscoelastic experimental results and the model used. 4. Conclusions Epoxy nanocomposite samples containing different weight percentages of non-functionalized and functionalized MWNT were fabricated in this study. The viscoelastic properties of the samples were evaluated. The hydroxyl groups on the MWNT surface were successfully increased the dispersion in the epoxy matrix. The dispersion degree of MWNT-OH significantly affects the improvement of viscoelastic properties. The addition of 0.5 wt.% MWNT and MWNT-OH to the epoxy matrix increased the storage modulus by 46% and 31.5% at room temperature. The rubbery modulus for 0.5 wt.% MWNT and MWNT-OH/epoxy composite was 9.1% and 71.8% higher than that of neat epoxy at 120 °C respectively. The weak interface between epoxy matrix and MWNT increased the loss modulus and damping of MWNT/epoxy composite specimen. It was demonstrated that the a-relaxation temperature increased with the addition of MWNT-OH. It seems that the hydrogen bonding between –OH groups with epoxy to be effective in the increase of viscoelastic properties. The addition of nanotubes did not have any effect on the b relaxation temperature. The parameters of Perez model showed a good correlation between viscoelastic experimental results and the model used. Acknowledgements The authors would like to express their gratitude to the personnel at Laboratoired’ingenierie des materiaux, ENSAM-Paris for conducting DMTA tests and SEM images. References [1] IiJima S. Hellical microtubules. Nature 1991;354:56–8. [2] IiJima S, Icihashi T. Single-shell carbon nanotubes of 1-nm diameter. Nature 1993;363:603–5. [3] Bethune DC, Kiang CH, Devries MS. Cobalt-catalyzed growth of carbon nanotubes with single-atomic-layer walls. Nature 1993;363:605–7. [4] Thostenson ET, Ren Z, Chow TW. Advances in the science and technology of carbon nanotubes and their composites: a review. Compos Sci Technol 2001;61:1899–912. [5] Jin Z, Promoda KPG. Dynamic mechanical behavior of melt-processed multiwalled carbon nanotube/poly (methacrylate) composite. Chem Phys Lett 2001;337:43–7. [6] Liao YH, Marietta-Tondin O, Liang Z, Zhang C, Wang B. Investigation of the dispersion process of SWNT/SC-15 epoxy resin nanocomposite. Mater Sci Eng 2004;A385:175–81. [7] Spitalsky´ Z, Krontiras CA, Georga SN, Galiotis C. Effect of oxidation treatment of multiwalled carbon nanotubes on the mechanical and electrical properties of their epoxy composites. Composites Part A 2009;40:778–83.
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